A central question in developmental biology concerns the control mechanisms governing selective gene transcription in chromatin. Considerable evidence indicates that transcriptional capability is dictated by chromatin structure and cell specific transcription factors. The Xenopus 5S system provides an example of a developmentally regulated gene for which an internal control region has been identified and a specific transcription factor has been isolated. We plan to investigate the nuclease sensitivity of the 5S genes as supercoiled DNA and as actively transcribed chromatin. Preliminary evidence indicates an S1 nuclease hypersensitive site at the boundary of the internal control region in supercoiled DNA which is similarly induced upon binding of the specific transcription factor. The altered conformation at this site may be related to the differential expression of the oocyte and somatic gene families. We hope to determine the biological significance of this altered conformation in terms of factor recognition. This research has implications regarding the fundamental mechanisms of gene control.